https://doi.org/10.1140/epjd/e2009-00009-9
Exact diagonalization of Hubbard clusters at finite temperatures
1
Advanced Materials Department, IPICYT, 78216 San Luis Potosí, Mexico
2
Institut für Theoretische Physik, Universität Kassel, 34132 Kassel, Germany
Corresponding author: a pastor@uni-kassel.de
Received:
19
September
2008
Published online:
24
January
2009
Finite temperature electronic and magnetic properties of small clusters
are investigated in the framework of the Hubbard model by using
exact diagonalization methods and by sampling the different cluster
topologies exhaustively. Results are discussed for
the specific heat C(T), magnetic susceptibility χ(T),
local magnetic moments μi(T), average magnetic moments
and spin-correlation functions γij(T).
Representative cluster sizes and band-fillings are considered
showing antiferromagnetic-like (AF) and ferromagnetic-like (FM)
behaviors. For half-band filling ν= N the susceptibility shows an
AF high-temperature behavior of the form χ≈1/(T + TN) from
which the cluster `Néel' temperature TN is derived.
In contrast, for ν= N + 1 a FM high-temperature behavior of
the form χ≈1/(T - TC) is found, where TC
can be interpreted as the cluster `Curie' temperature. In both cases
one also observes peaks in C(T), either at T≃TN
or T≃TC, which reflect the development of spin fluctuations
and the breakdown of the low-temperature short-range magnetic order.
The dependence of TN and TC on cluster size N and interaction
strength U/t is analyzed in terms of effective Heisenberg spin
interactions. Finally, the effects of temperature-induced structural
fluctuations are discussed.
PACS: 36.40.Cg – Electronic and magnetic properties of clusters / 75.10.Lp – Band and itinerant models
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2009